Polymeric watersplitting photocatalysts; a computational perspective on the water oxidation conundrum

A computational scheme to predict the thermodynamic ability of photocatalysts to drive both of the watersplitting half reactions, proton reduction and water oxidation, is discussed, and applied to a number of polymeric systems to explain their apparent inability to oxidise water. We predict that the poly(p-phenylene) (PPP) is thermodynamically unable to oxidise water and that PPP is hence unlikely to split water in the absence of an external electrical bias. For other polymers, however, for example carbon nitride, the lack of oxygen evolution activity appears kinetic in origin and hence a suitable co-catalyst could potentially transform them into true watersplitting photocatalysts

Polymer photocatalysts for water splitting: insights from computational modeling

Based on insights from computational chemistry calculations, the ability of polymers to act as water splitting photocatalysts for the production of renewable hydrogen from water and sunlight is discussed. Specifically, the important role of exciton dissociation in these materials is highlighted, as well as the possible microscopic origins of the experimentally observed changes in the photocatalytic activity of a polymer with increasing chain length or changing chemical composition. The reason why water oxidation, with polymeric photocatalysts, is difficult, and which polymer properties to target when developing new polymers for water splitting photocatalysis are, finally, also discussed

Shining a Light on s-Triazine-Based Polymers

The strong interplay between the structure and optical properties of conjugated s-triazine-based framework (CTF) materials is explored in a combined experimental and computational study. The experimental absorption and fluorescence spectra of the CTF-1 material, a polymer obtained through the trimerization of 1,4-dicyanobenzene, are compared with the results of time-dependent density functional theory and approximate coupled cluster theory (CC2) calculations on cluster models of the polymer. To help explain the polymer data, we compare its optical properties with those measured and predicted for the 2,4,6-triphenyl-1,3,5-triazine model compound. Our analysis shows that CTFs, in line with experimental diffraction data, are likely to be layered materials based around flat hexagonal-like sheets and suggests that the long-wavelength part of the CTF-1 absorption spectrum displays a pronounced effect of stacking. Red-shifted peaks in the absorption spectrum appear that are absent for an isolated sheet. We also show that the experimentally observed strong fluorescence of CTF-1 and other CTF materials is further evidence of the presence of rings in the layers, as structures without rings are predicted to have extremely long excited state lifetimes and hence would display negligible fluorescence intensities. Finally, subtle differences between the experimental absorption spectra of CTF-1 samples prepared using different synthesis routes are shown to potentially arise from different relative arrangements of stacked layers

Carbon Nitride Photocatalysts for Water Splitting: A Computational Perspective

We study the thermodynamic ability of carbon nitride materials to act as water splitting photocatalysts using a computational approach that involves a combination of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on cluster models of both triazine- and heptazine-based structures. We first use TD-DFT to calculate the absorption spectra of the different cluster models and compare these spectra to those measured experimentally and then calculate using DFT and TD-DFT the reduction potentials of the free electron, free hole, and exciton in these models. We predict that all classes of carbon nitride structures considered should thermodynamically be able to reduce protons and oxidize water. We further provide evidence for the hypothesis that the experimental lack of overall water splitting activity for pure carbon nitride arises from the fact that water oxidation is a four-hole reaction and hence very susceptible to competition with electron–hole recombination. Finally, we propose that the recently reported overall water splitting activity of carbon nitride loaded with polypyrrole nanoparticles arises from a junction formed at the interface of both materials, which assists in keeping electrons and holes apart

Visible‐Light‐Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts

Linear poly(p‐phenylene)s are modestly active UV photocatalysts for hydrogen production in the presence of a sacrificial electron donor. Introduction of planarized fluorene, carbazole, dibenzo[b,d]thiophene or dibenzo[b,d]thiophene sulfone units greatly enhances the H2 evolution rate. The most active dibenzo[b,d]thiophene sulfone co‐polymer has a UV photocatalytic activity that rivals TiO2, but is much more active under visible light. The dibenzo[b,d]thiophene sulfone co‐polymer has an apparent quantum yield of 2.3 % at 420 nm, as compared to 0.1 % for platinized commercial pristine carbon nitride

A dry and fully dispersible bacterial cellulose formulation as a stabilizer for oil-in-water emulsions

Supplementary material related to this article can be found, in theonline version, at: doi:https://doi.org/10.1016/j.carbpol.2019.115657Bacterial cellulose (BC) is an emerging alternative to plant cellulose in different applications. Several works demonstrated the potential of never-dried BC; however, envisioning real industrial applications, a dry product retaining its functional properties upon rehydration is preferable. A dry and completely redispersible formulation of BC with carboxymethyl cellulose (CMC) was prepared by Spray-drying. The obtained material showed a Zeta Potential of (-67.0±3.9) mV, a Dv(50) of (601±19.7) µm and was able to decrease the oil/water interface energy. The dry BC:CMC formulation was employed as stabilizer in oil-in-water emulsions, in parallel with commercial plant celluloses and Xanthan gum. The emulsions were monitored over time by optical microscopy and characterized by rheological measurements. BC:CMC effectively stabilized emulsions against coalescence and creaming, at a concentration of 0.50 % - contrarily to other commercial dry celluloses due to the Pickering effect and to the structuring of the continuous phase, as seen with Cryo-SEM.This study was supported by the Portuguese Foundation for Scienceand Technology (FCT) under the scope of the strategic funding of UID/BIO/04469 unit, COMPETE 2020 (POCI-01-0145-FEDER-006684) and BioTecNorte operation (NORTE-01-0145-FEDER-000004) funded by the European Regional Development Fund (ERDF) under the scope of Norte2020. FCT also supported this work by funding UID/EQU/00511/2019 unit, Project POCI-01-0145-FEDER-028715 funded by FEDER funds through COMPETE 2020 and by national funds (PIDDAC) throughFCT/MCTES, and Project NORTE-01-0145-FEDER-000005, funded by ERDF under the scope of NORTE 2020. The authors also acknowledgeFCT for the PhD scholarship SFRH/BD/115917/2016 (Daniela Martins) and for the contract based on “Lei do Emprego Científico” (DL 57/2016) (Berta Estevinho).info:eu-repo/semantics/publishedVersio

Dry Bacterial Cellulose and Carboxymethyl Cellulose formulations with interfacial-active performance: processing conditions and redispersion

Dry or powdered formulations of food additives facilitate transportation, storage, preservation and handling. In this work, dry formulations of bacterial cellulose and carboxymethyl cellulose (BC:CMC), easily redispersible and preserving the functionality of the never-dried dispersions are reported. Different processing parameters and their effect on the materials properties were evaluated, namely: (i) wet-grinding of BC (Hand-blender, Microcut Head Impeller, High-pressure Homogenizer), (ii) drying of BC:CMC mixtures (fast drying at130 °C and slow drying at 80 °C) and subsequent (iii) comminution to different particle sizes. The dispersibility of the obtained BC:CMC powders was evaluated, and their functionality after redispersion was assessed by measuring the dynamic viscosity, the effect in oil/water interfacial tension (liquidliquid system) and the stabilization of cocoa in milk (solidliquid system). The size of BC fibre bundles was of paramount relevance to its stabilizing ability in multiphasic systems. A more extensive wet-grinding of the BC fibres was accompanied by a loss in the BC:CMC functionality, related to the increasingly smaller size of the BC bundles. Indeed, as the Dv (50) of the wet BC bundles was reduced from 1228 to 55 µm, the BC:CMC viscosity profile dropped and the effect on interfacial tension decreased. This effect was observed both on the never-dried and dry BC:CMC formulations. On the other hand, the drying method did not play a major effect in the materials properties. In a benchmarking study, the BC:CMC formulations, at a low concentration (0.15%), had better stabilizing ability of the cocoa particles than several commercial cellulose products.Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10570-020-03211-9) contains supplementary material, which is available to authorized users.This study was supported by FCT under the scope of the strategic funding of UID/BIO/04469/2019 unit and BioTecNorte operation (NORTE-01-0145-FEDER000004) funded by the European Regional Development Fund under the scope of Norte2020-Programa Operacional Regional do Norte. Daniela Martins also gratefully acknowledges FCT for the PhD scholarship, reference SFRH/BD/115917/2016.info:eu-repo/semantics/publishedVersio

Urea-Modified Carbon Nitrides: Enhancing Photocatalytic Hydrogen Evolution by Rational Defect Engineering

The primary amine groups on the heptazine-based polymer melon, also known as graphitic carbon nitride (g-C3N4), can be replaced by urea groups using a two-step postsynthetic functionalization. Under simulated sunlight and optimum Pt loading, this urea-functionalized carbon nitride has one of the highest activities among organic and polymeric photocatalysts for hydrogen evolution with methanol as sacrificial donor, reaching an apparent quantum efficiency of 18% and nearly 30 times the hydrogen evolution rate compared to the nonfunctionalized counterpart. In the absence of Pt, the urea-derivatized material evolves hydrogen at a rate over four times that of the nonfunctionalized one. Since defects are conventionally accepted to be the active sites in graphitic carbon nitride for photocatalysis, the work here is a demonstrated example of defect engineering, where the catalytically relevant defect is inserted rationally for improving the intrinsic, rather than extrinsic, photocatalytic performance. Furthermore, the work provides a retrodictive explanation for the general observation that g-C3N4 prepared from urea performs better than those prepared from dicyandiamide and melamine. In-depth analyses of the spent photocatalysts and computational modeling suggest that inserting the urea group causes a metal-support interaction with the Pt cocatalyst, thus facilitating interfacial charge transfer to the hydrogen evolving centers

On the use of nanocellulose as reinforcement in polymer matrix composites

AbstractNanocellulose is often being regarded as the next generation renewable reinforcement for the production of high performance biocomposites. This feature article reviews the various nanocellulose reinforced polymer composites reported in literature and discusses the potential of nanocellulose as reinforcement for the production of renewable high performance polymer nanocomposites. The theoretical and experimentally determined tensile properties of nanocellulose are also reviewed. In addition to this, the reinforcing ability of BC and NFC is juxtaposed. In order to analyse the various cellulose-reinforced polymer nanocomposites reported in literature, Cox–Krenchel and rule-of-mixture models have been used to elucidate the potential of nanocellulose in composite applications. There may be potential for improvement since the tensile modulus and strength of most cellulose nanocomposites reported in literature scale linearly with the tensile modulus and strength of the cellulose nanopaper structures. Better dispersion of individual cellulose nanofibres in the polymer matrix may improve composite properties